Generally, a cutting tool such as a drill, an end mill or the like is fitted to a rotation spindle of a machine tool by using a chucking device. FIG. 14 shows by way of example a conventional chucking device of the foregoing type, and the chucking device 1 consists of a main body 2, a cylindrical press-fitting member 3 and a tightening ring 4. A plurality of male threads 6a are formed on the outer peripheral surface of a fore end part 5 of the chucking device 1, and a tapered hole 6b whose inner diameter is enlarged toward the foremost end thereof is formed inside of the fore end part 5.
A tapered portion 3a capable of being fitted to the tapered hole 6b is formed on the outer peripheral surface of the press-fitting portion 3. On the other hand, a hole 3b adapted to be fitted onto a shank of a cutting tool 7 is formed on the inner peripheral surface of the press-fitting portion 3.
A plurality of female threads 8 capable of being threadably engaged with the male threads 6a of the chucking device 1 are formed on the inner peripheral surface of the tightening ring 4, and a plurality of balls 9 are disposed on a fitting surface to the press-fitting member 3.
With the chucking device 1 constructed in the above-described manner, when the tightening ring 4 is rotated in the tightening direction, the press-fitting member 3 is displaced in the P arrow-marked direction so that the outer peripheral surface of the press-fitting portion 3 is thrusted against the tapered hole 6b of the chucking device by the wedging action, causing the diameter of the press-fitting portion to be reduced, whereby the inner peripheral surface 3b of the press-fitting portion 3 firmly holds a shank of the cutting tool 7 in the tightened state.
With the conventional chucking device 1 as mentioned above, however, since the foremost end part of the press-fitting portion 3 is offset from the inside tapered surface of the main body 2, the offset part of the press-fitting member 3 does not sufficiently exhibit the wedging effect, resulting in a magnitude of tightening force effective for the cutting tool 7 becoming short. Thus, there arises a problem that the cutting tool is vibratively displaced during a cutting operation.
Therefore, an object of the present invention is to provide a chucking device which assures that a shank portion of a cutting tool can firmly be held in the tightened state over the whole range of a press-fitting member.